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United States Patent |
5,737,474
|
Aoki
,   et al.
|
April 7, 1998
|
Semiconductor optical device
Abstract
A semiconductor optical device is provided which includes a semiconductor
substrate, a first core layer disposed on the substrate with a second core
layer being interposed between the substrate and the first core layer. The
second core layer has a lower refractive index than that of the first core
layer. A ridge-shaped optical waveguide region is formed in a top surface
of the first core layer. The width of the ridge-shaped optical waveguide
is modulated along a direction which coincides with an optical axis of the
semiconductor optical device. Further, the width of a bottom surface of
the ridge-shaped optical waveguide is selected not to be greater than 4
.mu.m over a whole length thereof.
Inventors:
|
Aoki; Masahiro (Kokubunji, JP);
Takahashi; Makoto (Kokubunji, JP);
Sato; Hiroshi (Kokubunji, JP)
|
Assignee:
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Hitachi, Ltd. (Tokyo, JP)
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Appl. No.:
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542840 |
Filed:
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October 13, 1995 |
Foreign Application Priority Data
Current U.S. Class: |
385/131; 385/132 |
Intern'l Class: |
G02B 006/10 |
Field of Search: |
257/94,96,98
372/45,46,50
385/108,129-132
|
References Cited
U.S. Patent Documents
Re31806 | Jan., 1985 | Scifres et al. | 372/50.
|
5555544 | Sep., 1996 | Walpole et al. | 372/50.
|
Other References
The Autumn Conference of the Institute of Electronics, Information and
Communication Engineers of Japan, C-303, Sep., 1994 Optics Letters, vol.
16, No. 5, pp. 306-308 (1991).
|
Primary Examiner: Ngo; John
Attorney, Agent or Firm: Antonelli, Terry, Stout, & Kraus, LLP
Claims
We claim:
1. A semiconductor optical device, comprising:
a semiconductor substrate;
a first core layer disposed on said substrate with a second core layer
being interposed between said substrate and said first core layer, said
second core layer having a lower refractive index than that of said first
core layer; and
a ridge-shaped optical waveguide region formed in a top surface of said
first core layer;
wherein width of said ridge-shaped optical waveguide region is changed
along a direction coinciding with an optical axis of said semiconductor
optical device and wherein width of a bottom surface of said ridge-shaped
optical waveguide does not exceed 4 .mu.m over a whole length thereof.
2. A semiconductor optical device according to claim 1,
wherein said second core layer is implemented in a superlattice structure
formed of materials differing from each other in respect to composition
thereof.
3. A semiconductor optical device according to claim 1,
wherein angles formed between a surface of said substrate and both side
walls of said ridge-shaped optical waveguide region, respectively, are
each smaller than 90.degree. inclusive.
4. A semiconductor optical device according to claim 1,
wherein each of side walls of said ridge-shaped optical waveguide region is
defined by a (111) A crystal plane.
5. A semiconductor optical device according to claim 1,
wherein said ridge-shaped optical waveguide region is formed by processing
a cladding layer of InP forming a part the waveguide region by using one
aqueous solution selected from a group which consists of an aqueous
solution of hydrobromic acid, a mixed aqueous solution of hydrobromic acid
and phosphoric acid and a mixed aqueous solution of hydrobromic acid and
acetic acid.
6. A semiconductor optical device according to claim 1,
wherein said ridge-shaped optical waveguide region is formed by processing
a cladding layer of InP forming a part of the waveguide region by using
one aqueous solution selected from a group which consists of an aqueous
solution of hydrochloric acid, a mixed aqueous solution of hydrochloric
acid and phosphoric acid, and a mixed aqueous solution of hydrochloric
acid and acetic acid.
7. A semiconductor optical device according to claim 1,
wherein said ridge-shaped optical waveguide region is formed by processing
a cladding layer of AlGaAs forming a part of the waveguide region by using
an aqueous solution of hydrofluoric acid.
8. A semiconductor optical device according to claim 1,
wherein said ridge-shaped optical waveguide region is formed by processing
a cladding layer of In.sub.0.51 G.sub.0.49 P forming a part of the
waveguide region by using one aqueous solution selected from a group which
consists of an aqueous solution of hydrochloric acid, a mixed aqueous
solution of hydrochloric acid and phosphoric acid, and a mixed aqueous
solution of hydrochloric acid and acetic acid.
9. A semiconductor optical device according to claim 1,
wherein said ridge-shaped optical waveguide region is formed by processing
a cladding layer of InGaAlP forming a part of waveguide region by using an
aqueous solution of hydrofluoric acid.
10. A semiconductor optical device according to claim 9,
wherein said second core layer is implemented in a superlattice structure
formed of a combination of materials selected from a group which consists
of combinations of InAsP and InP, InGaAsP and InP, InAlAs and InP,
InGaAlAs and InP, GaAs and InGaP, GaAs and InAsP, GaAs and InGaAsP, and
GaAs and AlGaAs.
11. A semiconductor optical device according to claim 1,
wherein width of said ridge-shaped optical waveguide region is narrowed at
a light emission end thereof when compared with the width of the remaining
portion of said ridge-shaped optical waveguide region.
12. A semiconductor optical device according to claim 1,
wherein said ridge-shaped optical waveguide region includes at least two
electrodes for application of a current or a voltage; and
wherein said electrodes are formed on a surface of said semiconductor
optical device at a side where said ridge-shaped optical waveguide is
formed.
13. A semiconductor optical device comprising:
an optical active region and at least an output region for an incident
light, wherein said output region for the incident light comprises:
a first core layer and a second core layer, said second core layer having a
lower refractive index than that of said first core layer; and
a ridge-shaped optical waveguide region formed on said first core layer,
wherein width of said ridge-shaped optical waveguide region is changed
along a direction coinciding with an optical axis of said semiconductor
optical device.
14. A semiconductor optical device comprising:
an optical active region and at least an output region for an incident
light, wherein said output region for the incident light comprises:
a first core layer and a second core layer, said second core layer having a
lower refractive index than that of said first core layer, wherein said
second core layer comprises a superlattice structure; and
a ridge-shaped optical waveguide region formed on said first core layer,
wherein width of said ridge-shaped optical waveguide region is changed
along a direction coinciding with an optical axis of said semiconductor
optical device.
15. A semiconductor optical device comprising:
an optical active region and at least an output region for an incident
light, wherein said output region for the incident light comprises:
a first core layer and a second core layer, said second core layer having a
lower refractive index than that of said first core layer; and
a ridge-shaped optical waveguide region formed on said first core layer,
wherein width of said ridge-shaped optical waveguide region is changed
along a direction coinciding with an optical axis of said semiconductor
optical device, and
wherein width of a bottom surface of said ridge-shaped optical waveguide is
smaller than width of a top surface of said ridge-shaped optical
waveguide.
16. A semiconductor optical device according to claim 1, 13, 14 or 15,
wherein said semiconductor optical device is a semiconductor laser device.
17. A semiconductor optical device according to claim 1, 13, 14 or 15,
wherein said semiconductor optical device is an optical amplifier.
18. A semiconductor optical device according to claim 1, 13, 14 or 15,
wherein said semiconductor optical device is an optical modulator.
19. A semiconductor optical device according to claim 1, 13, 14 or 15,
wherein said semiconductor optical device is an optical switch.
20. A semiconductor optical device according to claim 1, 13, 14 or 15,
wherein said semiconductor optical device is a photodetector.
Description
BACKGROUND OF THE INVENTION
The present invention generally relates to a semiconductor optical device
and, more particularly, to such a semiconductor optical device which can
find profitable and advantageous applications to optical communication
systems, optical networks and others.
In general, an input (incident)/output (emitted) beam of a semiconductor
optical device such as a semiconductor laser, optical amplifier, optical
modulator and the like has a spot size which is as small as on the order
of about 1/3 to 1/5 of the beam diameter in the optical fiber. Under the
circumstances, coupling of a laser beam to an optical fiber is generally
realized by using an expensive optical lens system in an effort to prevent
the optical coupling efficiency from degrading due to mismatch in the beam
diameter between the semiconductor optical device and the optical fiber.
However, in view of the prospect that many semiconductor optical devices
having simple structures will be able to be manufactured at lower costs in
the not-too-distant future, there will certainly arise a demand for the
capability of mounting the semiconductor optical devices more conveniently
and inexpensively without resorting to the expensive conventional optical
lens systems. As one of the measures for satisfying such demand, there can
be mentioned a method of enlarging the diameter of the laser beam emitted
or outputted from the semiconductor laser. In this conjunction, an
approach has been proposed and reported that a layer thickness and/or a
waveguide width of the laser active layer is controlled or modulated in
the direction along the optical axis of the laser device for thereby
enlarging or expanding the diameter of the emitted or outputted beam.
Certainly, such waveguide width control of the active layer can easily be
realized by employing a conventional lithography process. By contrast, the
control or modulation of the layer thickness requires in general a
technique for controlling the growth of the active layer in a substrate
surface by using, e.g. a selective growth process or the like.
Consequently, the manufacturing process becomes extremely complicate.
Additionally, it is noted that the characteristics of the laser device
manufactured through the complicated process such as mentioned above are
in actuality considerably inferior to those of the ordinary laser device.
As an attempt for evading such complexity, a combination of a double core
waveguide structure and waveguide width modulation has been studied,
although it is still at a theoretical level. However, because the
waveguide width is as large as on the order of 7 .mu.m, the attempt
mentioned above can not practically be applied to a semiconductor laser
device, an optical modulator and the like which require indispensably a
single transverse mode of operation at a low threshold value.
Parenthetically, the approaches and techniques for expanding the output
beam diameter of the semiconductor laser device are reported in "The
Autumn Conference of the Institute of Electronics, Information and
Communication Engineers of Japan, C-303, September, 1994", while the
technique concerning the combination of the double core waveguide
structure and the waveguide width control (modulation) is reported in
"Optics Letters", Vol. 16, No. 5, pp. 306-308, (1991).
SUMMARY OF THE INVENTION
In the light of the state of the art briefed above, it is an object of the
present invention to provide a novel and improved structure of an output
beam-expanded semiconductor laser device which can be implemented by an
extremely simple manufacturing method and which can ensure a high output
power as well as excellent temperature characteristics at a lower
threshold value.
Another object of the present invention is to provide a method of
manufacturing the same.
A further object of the present invention is to provide a semiconductor
optical device structure which can profitably and advantageously be
applied to a semiconductor laser, an optical amplifier, an optical
modulator, an optical switch, a photodetector of indiumphosphor series or
an integrated optical waveguide device constituted by integrating at least
two of these devices.
In view of the above and other objects which will become more apparent as
the description proceeds, there is provided according to a general aspect
of the present invention a semiconductor optical device of a structure
which can be implemented very easily by combining a double core waveguide
structure and modulation or control of a width of a ridge-shaped waveguide
and which can ensure expansion of the input/output beam without incurring
deterioration in the characteristics of the optical waveguide device such
as the semiconductor laser.
According to another aspect of the present invention, there is provided a
technique for further enhancing the characteristics mentioned above by
adopting a ridge structure of a reversed mesa shape and a superlattice
waveguide.
In the first place, description will be made of an aspect of the invention
directed to an output beam expanded semiconductor laser structure which
can be implemented by adopting a combination of a double core waveguide
structure and a waveguide width modulation (control) as well as a method
capable of easily manufacturing the same without deteriorating the
characteristics thereof.
Referring to FIG. 1A of the accompanying drawings, there are formed on an
n-type InP substrate 1, an auxiliary waveguide layer 2 of n-type InGaAsP
(having a compositional wavelength of 1.10 .mu.m) in a thickness of 1.0
.mu.m, a spacer layer 3 of n-type InP in a thickness of 0.3 .mu.m, an
active layer 4 of InGaAsP (having a compositional wavelength of 1.3 .mu.m)
in a thickness of 0.1 .mu.m, a cladding layer 5 of p-type InP in a
thickness of 2.0 .mu.m and a cap layer 6 of p-type InGaAs in a thickness
of 0.2 .mu.m sequentially in this order through conventional processes.
Subsequently, a laser structure including a ridge-shaped optical waveguide
having a vertical mesa structure (see FIG. 1A) is formed through a wet
etching process by using an aqueous solution of hydrochloric acid or a
mixed aqueous solution of hydrochloric acid and phosphoric acid and a
conventional laser manufacturing process. At this juncture, it is
important to note that the bottom surface of the ridge-shaped waveguide
has a width of 2.5 .mu.m and is tapered or constricted to 0.5 .mu.m in
width at the laser beam output or emission end. Thus, for the operation
with a wavelength of 1.3 .mu.m, the laser structure constitutes a single
transverse mode waveguide. The length of the tapered region in which the
ridge width is gradually changed is 150 .mu.m, whereas the whole length of
the ridge-shaped waveguide is 500 .mu.m. Incidentally, FIG. 1B shows
partially a cross section taken along a line A-A' in FIG. 1A, and FIG. 1C
is a sectional view taken along a line B-B' in FIG. 1A.
It has experimentally been established that the laser device as fabricated
in the structure described above exhibits improved characteristics at a
room temperature and under continuous oscillation condition such that the
threshold value is in a range of 8 to 10 mA and that the oscillation
efficiency is 0.40 W/A. Besides, even during operation at a
high-temperature of 85.degree. C., there characteristics can be obtained
to those of the conventional laser device such that the threshold value
falls within a range of 18 to 22 mA and that the oscillation efficiency is
0.30 W/A. These advantageous features of the laser device fabricated
according to the teachings of the invention can be explained by the fact
that the laser characteristics scarcely undergo deterioration due to
introduction of the beam expanding function because the basic structure of
the laser and the manufacturing processes therefor are substantially the
same as those adopted in the conventional laser devices known heretofore.
On the other hand, the spot size of the output laser beam emitted from the
front end face having the ridge width of 0.5 .mu.m can be expanded to 7.5
.mu.m which is about three times as large as the beam spot size of about
2.5 .mu.m at the rear end face having a ridge width of 2.5 .mu.m. This
effect can be explained by the fact that owing to the ridge-shaped
waveguide structure taught by the present invention, the action for
confining the light in the ridge-shaped waveguide region progressively
becomes feeble as the ridge width gradually decreases, whereby
distribution of the light intensity expands from the active layer located
immediately below the ridge-shaped waveguide region toward the underlying
auxiliary waveguide layer. In that case, the scattering of light rays can
sufficiently be suppressed by selecting the length of the tapered region
to be on the order of 150 .mu.m. Incidentally, coupling of the laser
device of the structure described above with a single-mode optical fiber
having a core diameter of 10 .mu.m has shown that a coupling loss of less
than 2 dB can be realized with the positioning accuracy of .+-.3 .mu.m in
the horizontal and vertical directions.
According to another important aspect of the invention, it is taught to
implement the ridge-shaped waveguide in a reversed mesa structure. FIGS.
2A to 2C are views showing schematically an output beam expanded laser
device incorporating a ridge-shaped waveguide region of a reversed mesa
structure formed by a wet etching process with an aqueous solution of
hydrobromic acid or a mixed aqueous solution of hydrobromic acid and a
phosphoric acid. In the case of this beam expanded laser device,
resistivity of the ridge-shaped waveguide region can be decreased by a
factor of 1/3 to 1/2 when compared with the vertical mesa structure shown
in FIG. 1A, as a result of which not only the threshold current can
correspondingly be lowered but also high-temperature characteristics can
be improved. Besides, because the optical confinement within the
ridge-shaped waveguide region of the reversed mesa structure is more
effective when compared with that of the vertical mesa structure shown in
FIG. 1A, the quantity of the laser light confined within the active layer
decreases more steeply than in the case of the vertical mesa structure
(FIG. 1A) for a given decrease of the ridge width, whereby the conversion
efficiency of the beam spot size can effectively be improved. This in turn
means that the ridge width modulated or controlled region (i.e., the
tapered region) required for realizing the desired conversion ratio can be
made shorter by implementing the ridge-shaped waveguide in the reversed
mesa structure, whereby scattering loss and radiation loss which the light
travelling the waveguide undergoes can be suppressed more effectively and
positively. Thus, further enhancement and improvement of the laser
characteristics can be expected.
In general, it is very difficult to allow a mixed polycrystal such as
InGaAsP to grow stably in a layer throughout a thickness greater than 1
.mu.m. A further aspect of the invention is directed to this problem.
FIGS. 3A to 3C show another structure of the semiconductor optical device
in which the auxiliary waveguide layer 2 of n-type InGaAsP having a
compositional wavelength of 1.10 .mu.m and in a thickness of 1.0 .mu.m is
realized in a superlattice structure 7 constituted by n-type InGaAsP
layers (having a compositional wavelength of 1.0 .mu.m) of 0.05 .mu.m in
thickness and n-type InP layers of 0.05 .mu.m in thickness with a
periodicity of twenty stacks. Using to this structure, there can easily be
realized an auxiliary optical waveguide layer of superlattice structure
having a stable compositional distribution and a preferably low refractive
index.
As can be seen from the foregoing, there are provided according to the
teachings of the present invention optical structures for the output beam
expanded laser device which can easily be implemented by combinations of
the double core waveguide structure and the waveguide width modulation or
control elucidated above as well as methods of manufacturing the same
without incurring any appreciable deterioration in the aimed
characteristics. Further, it can readily be understood from the foregoing
description that the characteristics of the beam-expanded laser device can
further be improved by incorporating the ridge-shaped waveguide of the
reversed mesa structure and/or the auxiliary optical waveguide
superlattice structure.
By the way, it is known that degraded operation of a short wavelength
laser, such as one designed for a wavelength of about 1 .mu.m or less in a
high output power state, is ascribable to crystal deterioration in an end
face where the optical density distribution is high. In this conjunction,
it is to important to note that according to the teachings of the
invention, the beam spot size can easily be expanded or enlarged. This, in
turn, means that the optical density at the emission or output end face
can significantly be lowered. Thus, the crystal in the emission end face
can be protected against deterioration, whereby the use life of the
semiconductor laser device can remarkably be lengthened, to another great
advantage.
It goes without saying that essentially the same advantageous actions and
effects as those described above can be obtained equally in the cases
where the teachings of the present invention are applied to an optical
amplifier, optical modulator, an optical switch, a photodetector or an
integrated optical waveguide device implemented by integrating at least
two devices selected from those mentioned above.
The above and other objects, features and attendant advantages of the
present invention will more easily be understood by reading the following
description of the preferred embodiments thereof taken, only by way of
example, in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the course of the description which follows, reference is made to the
drawings, in which:
FIG. 1A is an isometric view showing schematically a semiconductor optical
device manufactured according to one aspect of the present invention;
FIG. 1B is a fragmentary schematic sectional view of the same taken along a
line A-A' in FIG. 1A;
FIG. 1C is a fragmentary schematic sectional view of the same taken along a
line B-B' in FIG. 1A;
FIG. 2A is an isometric view showing schematically a semiconductor optical
device manufactured according to another aspect of the present invention;
FIG. 2B is a fragmentary schematic sectional view of the same taken along a
line A-A' in FIG. 2A;
FIG. 2C is a fragmentary schematic sectional view of the same taken along a
line B-B' in FIG. 2A;
FIG. 3A is an isometric view showing schematically a semiconductor optical
device manufactured according to yet another aspect of the present
invention;
FIG. 3B is a fragmentary schematic sectional view of the same taken along a
line A-A' in FIG. 3A;
FIG. 3C is a fragmentary schematic sectional view of the same taken along a
line B-B' in FIG. 3A;
FIG. 4A is an isometric view showing schematically a semiconductor optical
device according to a first embodiment of the invention during a
manufacturing step;
FIG. 4B is a view similar to FIG. 4A and shows the same during another
manufacturing step;
FIG. 4C is a similar view showing the same in a finished state;
FIG. 5A is an isometric view showing schematically a semiconductor optical
device according to a second embodiment of the invention during a
manufacturing step;
FIG. 5B is a view similar to FIG. 5A and shows the same during another
manufacturing step;
FIG. 5C is a similar view showing the same in a finished state;
FIG. 6A is an isometric view showing schematically a semiconductor optical
device according to a third embodiment of the invention during a
manufacturing step;
FIG. 6B is a view similar to FIG. 6A and shows the same during another
manufacturing step;
FIG. 6C is a similar view showing the same in a finished state;
FIG. 7A is an isometric view showing schematically a semiconductor optical
device according to a fourth embodiment of the invention during a
manufacturing step;
FIG. 7B is a view similar to FIG. 7A and shows the same during another
manufacturing step;
FIG. 7C is a similar view showing the same in a finished state;
FIG. 8A is an isometric view showing schematically a semiconductor optical
device according to a fifth embodiment of the invention;
FIG. 8B is a schematic top plan view of the same;
FIG. 9A is an isometric view showing schematically a semiconductor optical
device according to a sixth embodiment of the invention during a
manufacturing step; and
FIG. 9B is a similar view showing the same in a finished state.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Now, the present invention will be described in detail in conjunction with
what are presently considered as preferred or typical embodiments thereof
by reference to FIGS. 4 to 9. In the following description, it is to be
understood that such terms as "left", "right", "top", "bottom",
"upwardly", "downwardly", "vertical", "horizontal" and the like are words
of convenience and are not to be construed as limiting terms.
Embodiment 1
FIGS. 4A to 4C are views for illustrating a semiconductor optical device
according to a first embodiment of the invention. Referring to these
figures, there are formed a semiconductor substrate 11 of n-type (100)
InP, an auxiliary optical waveguide layer 12 of a superlattice structure
constituted by n-type InGaAsP layers (whose compositional wavelength is
1.10 .mu.m) each of 0.05 .mu.m in thickness and n-type InP layers each of
0.05 .mu.m in thickness with a periodicity of ten combinations, an n-type
InP spacer layer 13 of 0.5 .mu.m in thickness, a multiple quantum-well
active layer 14 constituted by InGaAsP layers (whose compositional
wavelength is 1.37 .mu.m) each having a thickness of 6.0 nm and serving as
a well layer and InGaAsP layers (whose compositional wavelength is 1.10
.mu.m) each having a thickness of 8 nm and serving as a barrier layer with
a periodicity of seven combinations, an upper optical guide layer 15 of
InGaAsP having a compositional wavelength of 1.10 .mu.m and a thickness of
0.05 .mu.m, a p-type InP cladding layer 16 of 1.7 .mu.m in thickness and a
p-type InGaAs cap layer 17 which is about 0.2 .mu.m thick which are
deposited sequentially on the substrate 11 in order the above listed. The
various layers mentioned above may be formed by resorting to conventional
techniques or processes known heretofore.
Subsequently, through a conventional process, the cap layer 17 is etched
for forming thereby a stripe structure having a width of 4.9 .mu.m and
extending in the direction ›011!, wherein a tapered stripe region is so
formed as to have a width of 2.9 .mu.m at a laser beam emission or output
end face. The length of the tapered stripe region is 100 .mu.m. In
succession, a ridge-shaped waveguide region having a cross-section of a
reversed mesa structure whose side walls are defined by (111) A planes,
respectively, is formed through a wet etching process by using a mixed
aqueous solution of hydrobromic acid and phosphoric acid. Refer to FIG.
4A. As a result of this, the width of a bottom surface of the ridge-shaped
waveguide region which represents the width of the active layer is 2.5
.mu.m while it is 0.5 .mu.m at a tip end of the tapered region.
Next, a silicon oxide film 18 is deposited in a thickness of 0.15 .mu.m
over the whole surface of the substrate through a conventional process,
which is then followed by formation of a polyamide resin film 19 over the
whole surface of the substrate. Thereafter, a silicon oxide film window is
formed on a top surface of the ridge, and after formation of an electrode
20, a device having a cavity length of 400 .mu.m inclusive of the tapered
region of the length of 100 .mu.m is cut out through a cleavage process.
Finally, a reflection film having a high reflectivity of 90% is deposited
on a rear end face of the ridge-shaped waveguide region which has a width
of 2.5 .mu.m, also by resorting to a conventional process.
It has experimentally been established that the semiconductor optical
device fabricated in the structure described above exhibits improved
oscillation characteristics at a room temperature under continuous
oscillation conditions such that the threshold value falls within a range
of 8 to 12 mA and that the oscillation efficiency is 0.45 to 0.51 W/A.
Furthermore, even in a high-temperature operation at 85.degree. C., there
can be obtained such preferred characteristics that the threshold value is
in a range of 16 to 22 mA and that oscillation efficiency is about 0.30 to
0.34 W/A. Besides, the spot size of the output laser beam from a front end
face having the ridge width of 0.5 .mu.m is expanded to 7.5 .mu.m, which
means that expansion of the beam diameter about three times as large as
the beam spot size of about 2.5 .mu.m at the rear end face of the ridge
having the width of 2.5 .mu.m can be achieved. Coupling of the
semiconductor optical device of the structure described above with a
single-mode fiber having a core diameter of 10 .mu.m without using any
optical lens has shown that a coupling loss less than 2 dB can be realized
with the positioning accuracy of .+-.3 .mu.m in the horizontal and
vertical directions. Further, evaluation of the long-term reliability of
the semiconductor optical device according to the instant embodiment of
the invention performed at a high temperature of 90.degree. C. has shown
that the use life of the device as estimated is 100,000 hours at the
shortest.
Embodiment 2
FIGS. 5A to 5C show a semiconductor optical device according to a second
embodiment of the present invention, which is directed to a high-power
laser device capable of oscillating at a wavelength of 1.48 .mu.m, which
device can be fabricated through procedure or processes substantially
similar to those described above in conjunction with the first embodiment.
A distorted InGaAsP multiple quantum well structure 21 having an emission
light wavelength of 1.48 .mu.m is implemented as the active layer through
a conventional process. With a view to realizing a stable high-power
single transverse mode, the laser device according to the second
embodiment of the invention is so implemented as to have a light emission
region of 2.2 .mu.m in width, a tapered region of 200 .mu.m in length and
a cavity length of 900 .mu.m, wherein a low reflectivity film having a
reflectivity of 3% and a high reflectivity film having reflectivity of 90%
are formed on both end faces of the device. It has experimentally been
established that the semiconductor laser device fabricated in the
structure described above exhibits improved oscillation characteristics at
a room temperature under the continuous oscillation condition such that
the oscillation threshold value is about 25 to 32 mA and that the
oscillation efficiency is 0.40 to 0.43 W/A. Further, a maximum output
power of 300 mW can be obtained. On the other hand, the spot size of the
output laser beam emitted from the front end face having the ridge width
of 0.5 .mu.m is 7.5 .mu.m. By implementing this laser device in a module,
there can be obtained a maximum module output power of 240 mW, reflecting
a low loss of 1.0 dB in the coupling with the fiber. Furthermore, it has
been found that by employing the device according to the instant
embodiment of the invention as an excitation light source for an
Erbium-doped fiber amplifier, excellent optical amplification
characteristics of smaller noise figure can be realized.
Embodiment 3
FIGS. 6A to 6C show a semiconductor optical device according to a third
embodiment of the present invention, which is implemented in the form of a
high power distribution feed-back type laser device designed to oscillate
at a wavelength of 1.55 .mu.m, and which can be fabricated through
procedure or processes substantially similar to those described above in
conjunction with the first embodiment. A distorted InGaAsP multiple
quantum well structure 21 having an emission light wavelength of 1.55
.mu.m and a .lambda./4 phase shift diffraction grating 32 having a period
of 241 nm are implemented in the active layer by a conventional process.
With a view to realizing a high output power and a stable single
transverse mode, the laser device according to the instant embodiment of
the invention is so implemented as to have a light emission region having
a width of 2.2 .mu.m, a tapered region having a length of 150 .mu.m and a
cavity length of 600 .mu.m, wherein a low reflectivity film having a
reflectivity of 1% and a high reflectivity film having a reflectivity of
90% are formed on both end faces of the device, respectively. It has
experimentally been established that the semiconductor laser device
fabricated in the structure described above exhibits improved oscillation
characteristics at a room temperature under the continuous oscillation
condition such that the oscillation threshold value is 15 to 18 mA and
that the oscillation efficiency is 0.35 to 0.40 W/A. Further, a maximum
output power of 150 mW can be obtained. On the other hand, the spot size
of the output laser beam emitted from the front end face having the ridge
width of 0.5 .mu.m is 7.5 .mu.m. By implementing this laser device in a
module, there could be obtained a maximum module output power of 240 mW,
reflecting a low coupling loss of 1.0 dB.
Embodiment 4
FIGS. 7A to 7C show a semiconductor optical device according to a fourth
embodiment of the present invention, which is directed to a semiconductor
laser device. Referring to these figures, there are formed on an n-type
(100) GaAs semiconductor substrate 11, a buffer layer 42 of n-type
In.sub.0.51 Ga.sub.0.49 P in a thickness of 2.0 .mu.m, an auxiliary
waveguide layer 43 of a super lattice structure constituted by n-type GaAs
layers each of 0.01 .mu.m in thickness and n-type In.sub.0.51 Ga.sub.0.49
P layers each of 0.09 .mu.m in thickness with a periodicity of ten stacks,
an n-type In.sub.0.51 Ga.sub.0.49 P spacer layer 44 of 0.5 .mu.m in
thickness, a single quantum well active layer 45 constituted by an
In.sub.0.17 Ga.sub.0.83 As layer of 6.0 nm in thickness which serves as a
well layer and an InGaAsP having a compositional wavelength of 0.70 .mu.m
and a thickness of 8 nm and serves as a barrier layer, an upper optical
guide layer 46 of InGaAsP having a compositional wavelength of 0.70 .mu.m
and a thickness of 0.05 .mu.m, a p-type In.sub.0.51 Ga.sub.0.49 P cladding
layer 47 of 1.7 .mu.m in thickness and a p-type GaAs cap layer 48 of 0.2
.mu.m thick, wherein the various layers mentioned above are deposited
sequentially on the substrate 11 in this order by resorting to
conventional techniques or processes.
Subsequently, through a conventional process, the cap layer 48 is etched
for forming thereby a stripe structure having a width of 4.6 .mu.m and
extending in the direction ›011!through a conventional process with a
tapered stripe region being formed so as to present a width of 2.9 .mu.m
at a laser beam emission or output end face. The length of the tapered
stripe region is 200 .mu.m. In succession, a ridge waveguide having a
cross-section of a reversed mesa shape having side walls defined by (111)
A planes is formed through a wet etching process by using a mixed aqueous
solution of hydrochloric acid and phosphoric acid, as can be seen in FIG.
7A. As a result of this, the width of a bottom surface of the ridge-shaped
region which represents the width of the active layer is 2.2 .mu.m while
it is 0.5 .mu.m at a tip end of the tapered region.
Next, a silicon oxide film 18 having a thickness of 0.15 .mu.m is deposited
over the whole surface of the substrate by a conventional method, which is
then followed by formation of a polyamide resin film 19 over the whole
surface of the substrate. Thereafter, a window of silicon oxide film is
formed on a top surface of the ridge, and after formation of electrodes, a
device having a cavity length of 900 .mu.m inclusive of the tapered region
having the length of 200 .mu.m is cut through a cleavage process. Finally,
a low reflection film having a reflectivity of 3% is formed on a front end
face of the ridge having a width of 0.5 .mu.m with a high reflection film
having a reflectivity of 90% being formed on a rear end face of the ridge
which has a width of 2.2 .mu.m. Thus, there is realized a high output
power semiconductor laser device which is designed to oscillate at a
wavelength of 0.98 .mu.m.
It has experimentally been established that the semiconductor laser device
fabricated in the structure described above exhibits improved oscillation
characteristics at a room temperature under the continuous oscillation
condition such that the oscillation threshold value is 12 to 15 mA and
that the oscillation efficiency is 0.60 to 0.70 W/A. Further, a maximum
output power of 400 mW can be obtained. The spot size of the output laser
beam emitted from the front end face having the ridge width of 0.5 .mu.m
is 7.0 .mu.m. By implementing this laser device in a module, there can be
obtained a maximum module output power of 300 mW, reflecting a low
coupling loss of 1.2 dB involved in the coupling with the optical fiber.
On the other hand, it is known that deterioration occurring in the high
power operation of the laser of the wavelength band mentioned above is due
to crystal deterioration at the emission end face having a high optical
density distribution. In this conjunction, it should be mentioned that in
the case of the laser device according to the instant embodiment of the
invention, the crystal degradation at the emission end face can remarkably
be suppressed even in the high power operation because the optical density
at the emission end face can be reduced by virtue of easy capability of
enlarging or expanding the beam spot size as mentioned previously.
Furthermore, it has been found that by using the semiconductor optical
device according to the instant embodiment of the invention as an
excitation light source for an Erbium-doped fiber amplifier, excellent
optical amplification characteristics of small noise figure can be
realized.
Parenthetically, it goes without saying that improvement or elongation of
the use life of the laser device owing to lowering of the optical density
at the emission end face by expanding the beam diameter can be assured for
all the high power semiconductor laser device having wavelengths not
longer than about 1 .mu.m.
Embodiment 5
FIGS. 8A and 8B show a semiconductor optical device according to a fifth
embodiment of the present invention, which is directed to a Mach Zender
type optical modulator. Referring to these figures, there are formed on an
n-type (100) InP semiconductor substrate 51, an auxiliary optical
waveguide layer 52 of a superlattice structure constituted by n-type
InGaAsP layers (having compositional wavelength of 1.10 .mu.m) each of
0.05 .mu.m in thickness and n-type InP layers each of 0.05 .mu.m in
thickness with a periodicity of ten stacks, an n-type InP spacer layer 53
of 0.5 .mu.m in thickness, a multiple quantum well structure (active
layer) 54 constituted by InGaAsP layers (whose compositional wavelength is
1.50 .mu.m) each having a thickness of 9.0 nm and serving as a well layer
and InP layers each having a thickness of 6 nm and serving as a barrier
layer, an upper optical guide layer 55 of InGaAsP having a compositional
wavelength of 1.15 .mu.m and a thickness of 0.05 .mu.m, a p-type InP
cladding layer 56 of 1.7 .mu.m in thickness, and a p-type InGaAs cap layer
57 having a thickness of 0.21 .mu.m. These various layers can be deposited
sequentially in the order mentioned above by resorting to conventional
growth techniques or processes. Subsequently, through a conventional
process, the cap layer 57 is etched in such a manner as shown in FIGS. 8A
and 8B. The waveguide extends in the direction ›011!. In succession, a
ridge-shaped waveguide having a cross-section of a reversed mesa shape
whose side walls are defined by (111) A planes, respectively, is formed
through a wet etching process by using a mixed aqueous solution of
hydrobromic acid and phosphoric acid, as can be seen in FIG. 8A. The
ridge-shaped waveguide region is so tapered that the width is decreased
from 1.5 .mu.m to 0.5 .mu.m at the emission end face, as can be seen in
FIG. 8B.
Subsequently, a silicon oxide film 57 having a thickness of 0.60 .mu.m is
deposited over the whole surface of the substrate through a conventional
process, which is then followed by formation of a polyamide film 58 over
the whole surface of the substrate. Thereafter, a window of silicon oxide
film is formed on a top surface of the ridge by an etch-back process, as
can be seen in FIGS. 8A and 8B. After formation of an electrode, a device
having a length of 1.4 mm is cut out by a cleavage process. Finally,
reflection films of a low reflectivity are formed on front and rear end
faces of the device to thereby manufacture a Mach Zender type optical
modulator.
It has experimentally been ascertained that the Mach Zender optical
modulator type device manufactured as mentioned above exhibits preferable
modulation characteristics. Furthermore, it has been found that the
overall insertion loss is as low as 7 dB, reflecting excellent optical
coupling characteristic with the optical fiber as well as the smooth
configuration of the side walls of the ridge waveguide. Besides, a
modulation band of 20 GHz can be realized, which reflects constriction or
narrowing of the width of the ridge. Additionally, optical data
transmission executed at a rate of 10 Gbits/second by using the optical
modulator device according to the instant embodiment shows that excellent
transmission can be achieved without being accompanied with any
appreciable deterioration of signal quality.
Embodiment 6
FIGS. 9A and 9B show yet another structure of the semiconductor optical
device according to a sixth embodiment of the present invention in which
an array of lasers for ten channels is formed on one and the same
substrate through procedures or processes substantially similar to those
described hereinbefore in conjunction with the first embodiment. More
specifically, a distorted InGaAsP multiple quantum well structure 61
having an emission light wavelength of 1.3 .mu.m is implemented as the
active layer by a conventional process. With a view to realizing a low
threshold value, the laser device according to the instant embodiment is
so implemented as to have a light emission region of 1.5 .mu.m in width
which is narrowed down to 0.3 .mu.m at the emission end. The tapered
region is 50 .mu.m in length with the cavity length of 150 .mu.m. High
reflector films having reflectivities of 80% and 90% are formed on both
end faces, respectively. It has experimentally been established that the
semiconductor lasers of all the channels exhibit improved oscillation
characteristics at a room temperature under the continuous oscillation
condition such that the oscillation threshold value is 1.3 to 1.5 mA and
that the oscillation efficiency is 0.45 to 0.47 W/A. The coupling
efficiency with optical fiber as achieved is 3 dB with the positioning
accuracy of .+-.3 .mu.m. It has been confirmed that the semiconductor
optical device according to the instant embodiment of the invention can be
utilized as a light source for optical fibers interconnecting which are
computer boards with excellent transmission characteristics improved
significantly in respect to delays in light emission and transmission
among others.
It is now apparent from the foregoing that according to the teachings of
the present invention, the ridge-shaped or ridge-loaded type optical
waveguide device can readily be realized which permits easy optical
coupling with an optical fiber and which can operate at low voltage and
current and nevertheless can assure high-speed operation characteristics.
Besides, the device according to the invention can be manufactured at a
surprisingly increased yield while assuring enhanced performance.
Additionally, applications of the semiconductor optical devices according
to the invention to optical communication systems will lead to
implementation of such systems at low cost with increased processing
capacity and extended range of communications.
Many features and advantages of the present invention are apparent form the
detailed description, and, thus, it is intended by the appended claims to
cover all such features and advantages of the system which fall within the
true spirit and scope of the invention. Further, since numerous
modifications and combinations will readily occur to those skilled in the
art, it is not intended to limit the invention to the exact construction
and operation illustrated and described.
By way of example, a mixed aqueous solution of acetic acid with hydrobromic
acid or hydrochloric acid may be employed, resulting in the same effects
as mentioned previously.
Accordingly, all suitable modifications and equivalents may be resorted to,
falling within the spirit and scope of the invention.
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